The invention relates to an exhaust gas system for an internal combustion engine having a thermoelectric power generator with a hot side and a cold side, the hot side being arranged on the exhaust gas system and being heatable by the exhaust gas of the internal combustion engine. A first heat exchanger having a first thermal conductivity is arranged between the hot side and the exhaust gas flow.
The relevant prior art includes, for example, published European patent application EP 1 475 532 A2, which discloses an internal combustion engine having an intake system and an exhaust gas system. The exhaust gas system and the intake system are connected together in a gas-conveying manner by way of an exhaust gas recirculation device. The exhaust gas recirculation device has a thermoelectric power generator, in order to recover electric power from the thermal energy of the exhaust gas. In order to guarantee an optimal supply of heat to the thermoelectric power generator by means of the exhaust gas recirculation line, valves are arranged in the exhaust gas recirculation line upstream and downstream of the thermoelectric power generator. These valves are actuated by a control unit as a function of the operating point. In order to obtain an optimal temperature differential between the hot side and the cold side of the thermoelectric power generator, the cold side of the thermoelectric power generator is connected to a coolant circuit.
Furthermore, it is known from published Japanese patent application JP 7012009 A2, on which this invention is based, to arrange the thermoelectric power generators on the exhaust gas system of an internal combustion engine. In this case, the exhaust gas line is divided into a plurality of parallel branches that convey the exhaust gas. Each of these branches has two thermoelectric power generators, as a result of which an optimal electric power generation under minimum exhaust gas back pressure conditions can be achieved. Furthermore, this branched exhaust gas system makes it possible to significantly reduce the amount of design space.
The drawback with the prior art arrangement of thermoelectric power generators is the relatively narrow speed or, more particularly, the load range of the internal combustion engine, in which the thermoelectric power generators optimally function, because the efficiency is very highly dependent on the temperature of the exhaust gas. Moreover, there is the risk of overheating if the internal combustion engine is operated in a high speed (load range) for a prolonged period of time.
An object of the present invention is to avoid the above-described drawbacks and to improve the efficiency of the electric power generation with thermoelectric power generators.
This and other objects are achieved with an exhaust gas system that is intended for an internal combustion engine and that includes a thermoelectric power generator with a hot side and a cold side, the hot side being arranged on the exhaust gas system and being heatable by the exhaust gas of the internal combustion engine. A first heat exchanger having a first thermal conductivity is arranged between the hot side and the exhaust gas flow. At least a second thermoelectric power generator is arranged on the exhaust gas system and includes a second heat exchanger having a thermal conductivity that is different from the first thermal conductivity.
In the simplest embodiment, the thermoelectric power generator having the heat exchanger with the lower thermal conductivity is arranged in close proximity to the internal combustion engine, and the second thermoelectric power generator having a heat exchanger with the higher thermal conductivity is arranged away from the internal combustion engine. This measure prevents the first thermoelectric power generator from being destroyed by overheating with an exhaust gas that is too hot. At the same time, the second thermoelectric power generator, including the heat exchanger that has the higher thermal conductivity and is operatively connected to the colder exhaust gas that has already been cooled, exhibits a higher efficiency at lower temperatures owing to the strategy according to the invention.
Preferably, in one embodiment, the exhaust gas system is characterized in that the exhaust gas system has at least a first and a second bypass, which are arranged one after the other in the direction of flow of an exhaust gas, the first thermoelectric power generator being arranged in the first bypass in the direction of flow of the exhaust gas, and the second thermoelectric power generator being arranged in the second bypass, and wherein a closure element for the exhaust gas is provided in at least one bypass.
Preferably, in the exhaust gas system, the first thermoelectric power generator has the heat exchanger with the lower thermal conductivity, and the second thermoelectric power generator has the heat exchanger with the higher thermal conductivity. This makes it possible to achieve a very good efficiency in the course of generating electric power.
In a second especially preferred design variant, the first and the second thermoelectric power generators are arranged in parallel to each other in the bypass, and wherein each thermoelectric power generator is heatable by the exhaust gas, at least one closure element for the exhaust gas being assigned to at least one thermoelectric power generator.
In another embodiment that makes it possible for the second especially preferred design variant to achieve a very good efficiency in the course of generating electric power, the exhaust gas of the internal combustion engine exhibits a colder or hotter temperature that is a function of an operating point. At least one closure element can be switched in such a way that hotter exhaust gas can be conveyed to the thermoelectric power generator having the heat exchanger with the lower thermal conductivity.
The use of a control unit is especially preferred. If the control unit is a control unit of the internal combustion engine, then this control unit also allows the characteristic parameters to be controlled in an advantageous manner.
In yet another embodiment that achieves an optimal temperature gradient between the hot side and the cold side of the thermoelectric power generator, the cold side of the thermoelectric power generator is integrated into a coolant circuit.
In an embodiment that achieves a very fast temperature rise of the thermoelectric power generators, the exhaust gas system has an additional closure element, wherein the additional closure element can be controlled in such a manner that all of the exhaust gas flows through the bypass.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.